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 FNG seeks IFR/IAB golden ratio 
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Joined: Sat Feb 25, 2017 11:58 am
Posts: 11
Long time lurker first time poster. Thanks to all for the wonderful knowledge nuggets so artfully hidden in plain sight here.

I'll clarify what I'm looking for, if the subject line doesn't make much sense:

An area ratio (if such a thing exists) to use as a starting point to balance the IFR and IAB. More or less, something that will get me a starting point to adjust to if one or the other requires change.

I ask because some of the posters here seem to be able to toss these numbers out like Kim Peek counting toothpicks. Are they all special snowflakes and I'm brushing against the shoulders of savants, or does the formula / rule of thumb I seek exist? Better just to sit in front of the Holley catalog with a calculator and smudge it out?

Thank you for your time, please be gentle with the cluebat.

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Tue May 09, 2017 12:19 am
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Joined: Mon Oct 21, 2013 12:15 am
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Location: San Diego, CA
I have developed formulas that I use for jetting, based on Horsepower. It is very accurate, and took a little time to figure it out. There is much more to it than that, but that is the basic idea. The numbers are different for types of fuel. I have also steered away from using jet numbers, opting for orifice size instead.

As far as air bleeds are concerned, I also use a formula that helps me see the effect of changing the bleed vs. the jet, or both.

Each carburetor type has a range that works, based on venturi, booster, throttle plate, length and exposure of T-slot, etc. Holley carburetor IABs work best in the range of .064"-.070" for the 4150, and .052"-low sixties on the Dominator. Holley has recently started using .073"+ on their IABs, but I am not convinced that this is a good thing.

As far as what you are asking, I do not personally use a ratio between IFR and IAB. As I stated before, there is a range that makes sense, and then anything beyond about .004" increase or decrease on IFR means that you can make a correction elsewhere. Sometimes, the circuits need a little help to get moving, and that is perfectly fine, as well. Keep in mind that every engine is different, and dictates tuning criteria.

Think of your cylinders like lungs. The lungs can take bigger gulps of air either with increased compression, or higher RPM (or both). This is why camshaft selection is so important. Low compression with big duration kills manifold vacuum, and smaller air bleeds in the idle circuit follow. That trend will follow all of the circuits to compensate, and then you end up with an inefficient engine with horrible AFRs. Manifold vacuum is the most important element in getting the fuel flow moving to begin with. Driving style, intended purpose of the vehicle, vehicle weight, gears, torque converter, compression, manifold style, plenum volume, and camshaft specs all play a role in which carburetor you select to begin with.

Carburetor selection is so important, because you want the signal to be strong enough so that you do not have to crutch the circuits in any way. You should see a definite change with the slightest adjustments. With air bleeds, .004" change should make a difference. .010" change is a good way to tell if you need to make a correction in the IFR or TSR. If you have to swing too far one way or the other on bleeds, restrictors, or jet, it is best to try a different size of carburetor. A properly tuned Holley-style carburetor should provide the highest vacuum at idle and minimal RPM drop in gear with .75-2 turns on mixture screws, strong but minimal overlap between idle, transition, accelerator, and main circuits, and provide the leanest AFR without hesitation, bucking, preignition, etc. Many people are surprised when they drive a properly tuned carbureted car. They will swear that there is fuel injection under the hood. A mild street small block should stay in the 600-650cfm range for daily driving and good manners, while a big block can tolerate 750-800cfm. These are merely guidelines, as there are so many possible scenarios but that is a good guideline to follow.

I know that Tuner and Mark have way more experience in these areas, and I am sure that they will add more to this, but this is my take on it. 99% of carburetor tuning is based on experience. A good carburetor builder/tuner has explicit notes on what worked on what combination. Carburetors don't stray too far from the beaten path, in most cases.

The best way to get a good baseline is to look up the specs on your carburetor, or map it out. Start your tuning at idle with the engine warmed up to operating temp, use a vacuum gauge to set the mixture screws, and then adjust your tune from there.


Tue May 09, 2017 10:52 am
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It's really a balance between Idle feed, idle air bleed, and t-slot size/t-slot restriction. And unfortunately there is nothing in stone, no two engines are alike, and in some cases like the Dominator castings you can get two that are completely different. With the idle feed selection you also have to look at what you need to accomplish. You can get roughly the same fuel with a smaller idle jet and bleed as you will with a larger combination, however the smaller combined will operate farther in the throttle opening before the circuit starts to diminish. You would look to do that with a two car arrangement, where you would need less fuel because you are supplying from 8 holes, but need it to provide fuel longer into the throttle opening. T-slot restrictions are used for oversized transition slots, some of the overseas baseplates and all of the Dominator style carbs use a slot that's too large. I recently built a new BLP billet with a 2.200 blade baseplate, The slot is only .180 long...


Tue May 09, 2017 10:57 am
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Joined: Thu May 08, 2014 7:47 pm
Posts: 235
Give a little here, take a little there. If you get the IFR to small the transition suffers, and it doesn't matter what you do, any where else, its not very tuneable, because the fuel is not there. As long as the IFR passes enough fuel the circuit is completley tuneable. If it does not pass the fuel, it doesnt matter what you do, because you cant tune, what is not there in the first place. Took me a little while to figure that out. Hope it makes sense.


Tue May 09, 2017 8:09 pm
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Joined: Sat Feb 25, 2017 11:58 am
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I suppose I should have offered some context... perhaps due to past success, I'm finding myself being asked more often to retune misbehaving hotrods for folks. Most, almost all really, aren't willing to redo a guessed / mashed up (certainly not engineered) engine combination that they paid megabucks for that doesn't perform as expected out of the box. Buying more parts is for the next go-round, the wad is shot, and they're asking me to make the best of what they have for a few hours of my time. Basically, ignition and fuel recalibration with existing hardware. Something like spit polishing a turd.

Mostly, I reduce egregious start, off/idle and cruise badness. I've been successful enough at improving otherwise awful combinations after a great deal of trial and error mixed with a liberal application of science and instrumentation (vacuum gauge, lambda meter, ears, nose, hands, feet and ass). I'm hoping for a shortcut to reduce the time spent unscrewing the screwups that arrive at my door, especially the ones that have already been deflowered by some other charlatan.

jmarkaudio hits closest with 'You can get roughly the same fuel with a smaller idle jet and bleed as you will with a larger combination, however the smaller combined will operate farther in the throttle opening before the circuit starts to diminish.' I understand this principle; after I've sorted very low speed, if I want to move the transition point upward (by using a smaller IAB of some percentile area change), what corresponding percentile area change should I make to the IFR or better yet how to solve for the approximate IFR?

I'm not so presumptuous to expect a perfect answer that covers every base, more like a calibrated wet thumb in the air for an idea of direction and speed, at least better than what I'm doing now.

What you're all saying makes good sense, and isn't terribly surprising: experience counts, keep comprehensive notes of your accomplishments and failures. I guess I'll just soldier on and discover trends on my own, thank you for your time.

One last question, does anyone know of tuning database software to track this stuff, or should I just spin my own?

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Tue May 09, 2017 11:32 pm
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Joined: Mon Oct 21, 2013 12:15 am
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Location: San Diego, CA
Using your current AFR and Desired AFR, convert both to LAMBDA. Then, calculate the current area of your jet, air bleed, etc. Depending on where you want your ratio to go (up or down) will dictate the area of change in percent, plus or minus. If you want your LAMBDA to increase by .05, then you add 5% area to your jet, or decrease the area of your air bleed by 5%. Use the square root of your new area divided by π multiplied by 2 to get your new diameter. You can build the formula yourself in Excel, and literally have the formula at your fingertips on your phone. I can calculate jetting, air bleeds, metering rods, etc., in seconds on my phone.
As you know, changing the air bleed not only richens or leans the affected circuits, but also affect the timing of the circuits.


Wed May 10, 2017 1:04 am
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Joined: Sat Feb 25, 2017 11:58 am
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Revved Up wrote:
Using your current AFR and Desired AFR, convert both to LAMBDA. Then, calculate the current area of your jet, air bleed, etc. Depending on where you want your ratio to go (up or down) will dictate the area of change in percent, plus or minus. If you want your LAMBDA to increase by .05, then you add 5% area to your jet, or decrease the area of your air bleed by 5%. Use the square root of your new area divided by π multiplied by 2 to get your new diameter.

I think I understand it now... however, I understand that a numeric lambda increase is a leaner condition.

Shouldn't that read 'If you want your LAMBDA to increase by .05, then you subtract 5% area from your jet, or increase the area of your air bleed by 5%', or am I backwards?

Revved Up wrote:
You can build the formula yourself in Excel, and literally have the formula at your fingertips on your phone. I can calculate jetting, air bleeds, metering rods, etc., in seconds on my phone.

I should see if my programmer buddy will make an Android app for this, Open Office does not like my phone.

Revved Up wrote:
As you know, changing the air bleed not only richens or leans the affected circuits, but also affect the timing of the circuits.

Agreed. Circuit timing, coupled to lambda, is the source of my consternation.

Thank you very much for the math, it will help immensely.

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Wed May 10, 2017 2:03 pm
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Joined: Mon Oct 21, 2013 12:15 am
Posts: 226
Location: San Diego, CA
You are correct. I should have proof-read that. My thinking was enrichment, and I was very tired when I wrote it.

Hope that it helps you.


Wed May 10, 2017 2:36 pm
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Joined: Fri Feb 08, 2013 2:37 pm
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For IABs I always use the L/D ratio of a Unicorn Horn, for MABs I divide that by the average diameter of Pixie Dust.

Perfect right outta the box, every time.


Thu May 11, 2017 2:47 pm
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Joined: Sat Feb 25, 2017 11:58 am
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Well, that must explain my results, no wonder I'm so far off! Thanks for the tip!

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Thu May 11, 2017 3:31 pm
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